How is the zero level of the drum liquid level defined?
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This post was last edited by mainsj on 2010-8-21 at 11:53. I would like to ask the experienced members: How is the zero point for the liquid level in a boiler’s drum defined? Is it 180 mm below the centerline of the drum? Are there any guidelines or standards?
The normal water level (NWL) of the drum refers to the height at which the water level in the drum should be maintained during the normal operation of the boiler. This is generally referred to as the zero water level, which is located below the centerline of the drum. Different boiler manufacturers specify different values for this level; it cannot be determined arbitrarily. (Note that in some cases the zero water level coincides with the centerline of the drum, so it is necessary to refer to the manufacturer’s drawings.)
From a safety perspective, having a higher drum water level and storing more water is beneficial for safe operation and preventing the boiler water from vaporizing as it enters the downcomer. However, in order to obtain steam of satisfactory quality, the steam-water mixture entering the drum must be properly separated into steam and water. Only when there is sufficient steam space in the drum can the steam-water separation device inside it function properly, resulting in a relatively satisfactory separation effect. Due to the heat dissipation of the level gauge, the water inside it is at a lower temperature and therefore has a higher density, while the boiler water in the drum is at a higher temperature and thus has a lower density. In some boilers, the steam-water mixture enters the drum below the water level, resulting in a lower density of the water in the drum. This causes the actual water level in the drum to be significantly higher than the level indicated by the gauge. Therefore, to ensure sufficient steam space, most medium-pressure and high-pressure boilers specify 150 mm below the drum centerline as the zero level for the water level gauge. Since the steam and water densities in ultra-high pressure and subcritical pressure boilers are more similar, steam-water separation is more difficult. Moreover, the difference between the temperature of the water in the boiler drum and the temperature of the water in the level gauge is greater in such boilers. To ensure effective steam-water separation, a larger steam space is required. Therefore, for ultra-high pressure and subcritical pressure boilers, the zero level of the water gauge is specified to be 200 mm below the centerline of the drum.